Aqueous dispersions containing functional materials have been widely used for agricultural chemicals such as herbicides and insecticides and drugs such as anticancer drugs, antiallergic drugs and anti-inflammatory drugs as functional materials. Meanwhile, coloring materials such as ink and toner containing a colorant in a form of solid particles are well known. In recent years, digital printing technologies represented by electrophotography and inkjet printing have been making great progress, and the significance of these technologies as an image formation technology is recognized more and more in office and home.
Among them, the inkjet technology has remarkable features such as compactness and low power consumption as a direct recording method. In addition, image quality has been rapidly improved owing to refinement of nozzles and the like. One example of the inkjet technology is a method in which ink supplied from an ink tank is heated by a heater in a nozzle to form a bubble therein by boiling, and the ink is discharged from the nozzle to form an image on a recording medium. Another method is a method in which a piezo element is vibrated to discharge ink from a nozzle. Since an aqueous dye solution is usually used in ink used in these methods, bleeding may occur when colors are superimposed, and a phenomenon called feathering may occur along paper fibers at a recording location on the recording medium. For the purpose of alleviating these problems, use of pigment dispersion ink is proposed (U.S. Pat. No. 5,085,698). However, many improvements are still desired.